The present invention is directed to methods and apparatus that use electrostatic and/or electromagnetic fields to enhance the process of spray forming preforms or powders. The present invention also describes methods and apparatus for heat transfer using non-equilibrium plasmas and for atomization.
Spray forming is a process by which a stream of molten metal is atomized by a gas stream impinging upon it. The resulting atomized droplets are then directed to a target by the gas stream, or the resulting atomized droplets are cooled to form a powder. Producing powders by typical prior spray forming methods results in a yield loss of 10-15%, and much of the loss is associated with powder being trapped in various areas of the apparatus rather than being delivered to the collection vessel during the process. In producing solid workpieces, known as preforms, typical prior spray forming methods result in a yield loss of 25-40%, and a significant portion of the loss is usually caused by over-spray and particles bouncing off the surface due to their angular impact relative to the normal of the preform surface. Various methods have been described to recover and reuse overspray powder, such as, for example, U.S. Pat. No. 5,649,993, but these are not wholly satisfactory.
Because many powders and preforms are susceptible to damage to their chemical structure by air and oxygen, they are often produced in a shield gas environment of nitrogen or argon. The flow of shield gas, however, must be turned off to allow workers to enter the chamber for cleanup, changeover and maintenance. Thus, any powder or preform remaining in the chamber becomes contaminated and unusable when air and oxygen enter the spray forming apparatus after the flow of shield gas is turned off.
Previously, gas streams or jets have been used to direct the path of the particles involved in the spray forming process. The gas streams typically consist of argon or nitrogen as the means of directing the particles, and heat is removed from the workpiece through conduction or convection.
Current processes for making powder metal products, particularly in materials used for critical aerospace applications, use a conventional gas atomizing process. In this process, high-pressure gas is directed at a molten metal stream to break it into smaller droplets. The droplets solidify as powder. For critical applications, the resultant powder is then blended with batches of powder from other small melts. The blend is screened to a small mesh size (325 mesh), canned and consolidated by extrusion into product suitable for manufacture into an aircraft component. This method of manufacture is not efficient because several small melts are required for blending, melts are made in conventional ceramic lined furnaces and hence result in oxide contamination, several powder handling operations offer opportunity for contamination, and many steps in the process make the production operation costly.
Heat transfer using non-equilibrium plasmas has heretofore been poorly understood and often incorrectly or inefficiently applied. There is a need in the art for methods and apparatus that improve the yield and quality of powders and preforms produced by spray forming. The present invention is directed to these, as well as other, important ends.
The present invention overcomes the limitations of the conventional powder process by permitting a significantly larger melt to be manufactured to powder, thereby eliminating the blending steps. They also are melted and atomized in a ceramicless system, thereby minimizing the contamination from the furnace linings. They are atomized in vacuum, thereby eliminating the need for screening and handling. They can either be containerized and sealed in a vacuum or rapidly solidified to form a solid preform in vacuum, thereby eliminating sources of handling and hence possible contamination. Finally, the present invention will have considerably fewer handling steps than conventional powder making, and thus will be more cost effective.
In one embodiment, the present invention describes apparatus comprising dispensing means, collecting means, and means for directing molten particles from the dispensing means to the collecting means comprising an electrostatic field and/or an electromagnetic field. Optionally, the apparatus may further comprise atomization apparatus and/or non-equilibrium heat transfer apparatus.
In another embodiment, the present invention describes spray forming methods comprising directing molten particles from dispensing means to collecting means by producing an electrostatic field and/or electromagnetic field between the dispensing means and the collecting means. Optionally, the apparatus may further comprise atomization apparatus and/or non-equilibrium heat transfer apparatus.
In another embodiment, the present invention is directed to apparatus comprising a melt chamber that comprises at least one orifice; a means for expelling a molten material through the at least one orifice in the melt chamber; and a means for applying a rapid electrostatic charge to the molten material. Preferably, the means for forcing the molten material through the at least one orifice in the melt chamber is a mechanical or electromechanical actuator or a pressure means. In a preferred embodiment, the apparatus further comprises a means for cooling the molten particle. Preferably, the means for cooling the molten particle comprises a means for generating a non-equilibrium plasma.
In another embodiment, the present invention describes methods for forming particles comprising producing a first molten particle; and applying a rapid electrostatic charge to the first molten particle, wherein the rapid electrostatic charge causes the first molten particle to form at least one smaller second particle. Preferably, the first molten particle is expelled through at least one orifice in the melt chamber via mechanical means or by a pressure means. In a preferred embodiment, the at least one smaller second molten particle is cooled, preferably by a non-equilibrium plasma.
In another embodiment, the present invention is directed to apparatus for transferring heat between a heat-transfer device and a workpiece comprising the heat-transfer device, wherein the heat-transfer device is electrically charged or held at a potential; the workpiece, wherein the workpiece is mechanically separate from the heat-transfer device; and means for transferring heat between the workpiece and the heat-transfer device comprising a means for generating a non-equilibrium plasma. The heat-transfer device can be either a heat sink or a heat source.
In yet another embodiment, the present invention is directed to methods of transferring heat between a heat-transfer device and a workpiece comprising producing a non-equilibrium plasma capable of transferring heat between the heat-transfer device and the workpiece, wherein the heat-transfer device is electrically charged or held at a potential, and wherein the heat-transfer device is mechanically separate from the workpiece. The heat-transfer device can be either a heat sink or a heat source.
Accordingly, in various embodiments, non-equilibrium plasmas are advantageously employed to effect optimal heat transfer, and the non-equilibrium plasma must act with a heat sink/source that has a thermal conductivity capable of removing the desired quantity of heat. While two or more electrodes have been used in the past to produce a plasma in a region of high heat, such as a weld zone, so that the plasma would serve to conduct heat outward from the weld zone, thereby increasing the surface area for heat, embodiments of the present invention are directed to the discovery that a non-equilibrium plasma may be used to introduce heat into a workpiece as well as from a workpiece. It has further been surprisingly discovered that under the correct conditions a non-equilibrium plasma can be used to efficiently transfer heat in a vacuum.
The novel methods of the present invention are particularly useful in preparing any metal article, such as articles for gas turbine engines, including, for example, airfoils, blades, discs and blisks.
Accordingly, in one aspect, there is provided according to the present invention an apparatus comprising: a dispensing means; a collecting means; and a means for directing a molten particle from the dispensing means to the collecting means comprising at least one of an electrostatic field or an electromagnetic field. In another aspect is provided the apparatus described above, wherein the means for directing the molten particles from the dispensing means to the collecting means comprises an electrostatic field or an electromagnetic field. The apparatus may further comprise at least one magnetic coil, and may also further comprise a means for charging the molten particles. In one embodiment, the means for charging the molten particles may comprise a thermionic emission source or a tribocharging device. The dispensing means of the apparatus may be a gas atomizer, and may further comprise a means for transferring heat from the molten particles. The means for transferring heat from the molten particles may comprise gas conduction and/or convection and/or a non-equilibrium plasma.
In another aspect, there is provided according to the present invention an apparatus comprising: a dispensing means; a collecting means; and a means for directing a molten particle from the dispensing means to the collecting means comprising at least one of an electrostatic field or an electromagnetic field, and further comprising a means for transferring heat from the collecting means. The means for transferring heat from the collecting means may comprise a means for generating a non-equilibrium plasma. In a particular aspect, the means for transferring heat from the molten particles comprises a first heat sink, wherein the first heat sink is electrically charged or held at a potential; and a means for transferring heat from the molten particles to the first heat sink comprising a means for generating a non-equilibrium plasma. The non-equilibrium plasma may be a glow discharge or a cold corona discharge.
In another aspect, there is provided according to the present invention an apparatus comprising: a dispensing means; a collecting means; and a means for directing a molten particle from the dispensing means to the collecting means comprising at least one of an electrostatic field or an electromagnetic field, and further comprising a means for expelling the molten particle through at least one orifice in the dispensing means; and a means for applying a rapid electrostatic charge to the molten material. The means for expelling the molten particle through the at least one orifice may comprise a mechanical or electromechanical actuator. In one aspect, the means for expelling the molten particle through the at least one orifice may be a pressure means that produces a pressure in the dispensing means that is greater than the pressure on the outside of the dispensing means. The pressure means may cause interrupted flow of the molten particle from the dispensing means. The rapid electrostatic charge may be an arc discharge or an electron beam.
In another aspect, the present invention provides for a spray forming method comprising directing molten particles from a dispensing means to a collecting means by producing at least one of an electrostatic field or an electromagnetic field between the dispensing means and the collecting means. The electromagnetic field may be produced by, for example, means comprising at least one magnetic coil. The method according to this aspect of the invention may further comprise charging the molten particles. Charging the molten particles may be accomplished, for example, using a thermionic emission source or a tribocharging device. In one aspect, the dispensing means may be a gas atomizer. According to this aspect of the invention, the method may further comprise transferring heat from the molten particle. Transferring heat from the molten particles may be accomplished, for example, by gas conduction and/or convection and/or non-equilibrium plasma. In another aspect, the method of the invention further comprises producing a second electromagnetic field. According to the invention, the method may further comprise transferring heat from the collecting means, which may be by a non-equilibrium plasma.
In another aspect, the present invention provides for a spray forming method comprising directing molten particles from a dispensing means to a collecting means by producing at least one of an electrostatic field or an electromagnetic field between the dispensing means and the collecting means, further comprising applying a rapid electrostatic charge to the molten particle, wherein the rapid electrostatic charge causes the molten particle to form at least one smaller molten particle. In a particular aspect, the rapid electrostatic charge may be an arc discharge or an electron beam. In another aspect, the method of the invention may further comprise transferring heat from the molten particle comprising producing a non-equilibrium plasma that transfers heat from the molten particle to a first heat sink, wherein the first heat sink is electrically charged or held at a potential. The non-equilibrium plasma may be a glow discharge or a cold corona discharge.
In another aspect, the invention is directed to an apparatus comprising a melt chamber comprising at least one orifice; a means for forcing a molten material through the at least one orifice in the melt chamber; and a means for applying a rapid electrostatic charge to the molten material. The rapid electrostatic charge may be an arc discharge or en electron beam. The apparatus of the invention may further comprise a means for cooling the molten material. In a particular aspect, the means for cooling the molten material may comprise a first heat sink, wherein the first heat sink is electrically charged or held at a potential; and a means for transferring heat from the molten material to the first heat sink comprising a means for generating a non-equilibrium plasma. The non-equilibrium plasma may be a glow discharge or a cold corona discharge.
In another aspect, there is provided a method for atomizing a particle comprising producing a first molten particle; applying a rapid electrostatic charge to the first molten particle, wherein the rapid electrostatic charge causes the first molten particle to form at least one smaller second molten particle. According to the method of the invention, the first molten particle may be produced by melting a material in a melt chamber, and expelling the first molten particle through at least one orifice in the melt chamber. The rapid electrostatic charge may be an arc discharge or en electron beam. The method of the invention may further comprise cooling the second molten particle by producing a non-equilibrium plasma that transfers heat from the second molten particle to a first heat sink, wherein the first heat sink is electrically charged or held at a potential. The non-equilibrium plasma may be a glow discharge or a cold corona discharge.
In another aspect, the invention provides for an apparatus for transferring heat between a first heat-transfer device and a workpiece comprising a first heat-transfer device, wherein the first heat-transfer device is electrically charged or held at a potential, and wherein the first heat-transfer device is a heat sink or a heat source; a workpiece, wherein the workpiece is mechanically separate from the first heat-transfer device; and means for transferring heat between the workpiece and the first heat-transfer device comprising a means for generating a non-equilibrium plasma. The non-equilibrium plasma may be a glow discharge or a cold corona discharge. The apparatus of the invention may further comprise an external means for generating or maintaining the non-equilibrium plasma. The external means for generating or maintaining the non-equilibrium plasma may be a thermionic emission, an RF electromagnetic radiation, an electromagnetic radiation, a magnetic field or an electron beam. The first heat-transfer device of the apparatus of the invention may comprise a plurality of heat-transfer devices. In a particular aspect, the apparatus of the invention may further comprise a second heat-transfer device that may be mechanically and electrically separate from the first heat-transfer device, wherein the second heat-transfer device is a heat sink or a heat source, and wherein the potential between the first heat-transfer device and the second heat-transfer device produces a non-equilibrium plasma.
In another aspect is provided a method for transferring heat between a first heat-transfer device and a workpiece comprising producing a non-equilibrium plasma that transfers heat between the first heat-transfer device and the workpiece, wherein the first heat-transfer device is electrically charged or held at a potential, wherein the first heat-transfer device is mechanically separate from the workpiece, and wherein the first heat-transfer device is a heat sink or a heat source. The non-equilibrium plasma may be a glow discharge or a cold corona discharge. The method may further comprise generating or maintaining the non-equilibrium plasma via an external means. In an aspect, the external means for generating or maintaining the non-equilibrium plasma comprises a thermionic emission, an RF electromagnetic radiation, an electromagnetic radiation, a magnetic field or an electron beam.
In another aspect, the invention provides for a preform produced by the methods of the invention. The preform of the invention may be a near net preform. There is also provided an article of manufacture produced by the method of the invention.
These and other aspects of the present invention will become more apparent from the following detailed description.